This invention relates to the coated back (CB) component of pressure-sensitive record elements, commonly referred to as carbonless copy paper. More particularly, this invention relates to high solids content sodium carboxymethyl cellulose (CMC)-based coating formulations with efficient coating viscosity and paper coated with the formulations.
Carbonless copy paper comprises two or more juxtaposed sheets. The back surfaces of the sheets have a coating containing a color-forming material, often referred to as a coated back or "CB". The coating consists of a continuous matrix or microcapsules containing the color-forming material. The front surfaces of the receiver sheets, which in use are placed in contact with the back surfaces of the overlaid sheets, are coated with a composition containing a developer component reactive with the color-forming material. Under the application of localized pressure to the top sheet, the coatings containing the color-forming materials are ruptured thereby releasing the color-forming material to contact and react with the co-reactant or developer coating on the receiver sheets. A visible colored image is produced in areas corresponding to the locations where pressure has been applied to release the color-forming material. Therefore, the pressure applied to the top sheet causes a corresponding mark on the front of each of the other sheets of the manifold set.
The prior art discloses processes for production of coatings containing color-forming materials (also referred to as color precursors or leuco dyes) dissolved in a solvent. Previous patents, for example, U.S. Pat. No. 4,397,483, 4,082,688, and GB 1,280,769, disclose CB coatings for pressure-sensitive recording materials comprising a carboxymethylcellulose precipitate containing an oil-like solvent and a color-forming reactant. The oil containing the color-forming reactant is emulsified in an aqueous solution of sodium carboxymethylcellulose, and a metal salt is added to cause precipitation of the cellulose with droplets of the oil dye solution contained therein as an isolated phase. The resulting emulsion is coated on the back of the top sheet of a carbonless copy system and dried. Often, cross-linking resins are added to the emulsion to toughen the resulting coating.
While these coatings are commercially successful, high solids CB compositions based on previous embodiments of this technology display poor stability and unmanageable viscosities too high for coating equipment and methods now in use. Low solid content CB coatings (e.g., 20% non-aqueous components) require high energy comsumption and extensive drying time. Accordingly, it would be desirable to provide a high percent solids content CB coating with an efficient coating viscosity.
It is not possible to merely decrease the water content, thereby increasing the solids content of the known CMC-based coating composition, because this results in serious rheological problems. Aqueous solutions of CMC are characterized by viscosities which increase exponentially with increased CMC concentration. Accordingly, any advantage achieved by simply increasing the CMC content, thereby producing a coating composition which may be dried more rapidly, is offset by that composition's unacceptably high viscosity and resulting poor workability.
It is an object of this invention to provide an improved high solids content CB coating with low viscosity for better coating onto paper and other substrates. Another object is to increase drying efficiencies by providing a coating composition with a higher solids content and less water. Still another object is to increase savings in labor and energy by producing low water-content coating compositions. Another object is to achieve improved imaging on computer and dot matrix printers using pressure-sensitive paper by providing a CB paper which has improved color-former release efficiency. Another object is to produce CB coatings which are whiter in appearance. Yet another object is to decrease paper substrate distortion by wetting of fibers. Another object is to provide a CB coating that can be applied on a papermaking machine.